Photolithography is commonly used during formation of integrated circuits on semiconductor wafers. More specifically, a form of radiant energy (such as, for example, ultraviolet light) is passed through a radiation patterning tool and onto a radiation-sensitive material (such as, for example, photoresist) associated with a semiconductor wafer. The radiation patterning tool can be referred to as a photomask or a reticle. The term “photomask” traditionally is understood to refer to masks which define a pattern for an entirety of a wafer, and the term “reticle” is traditionally understood to refer to a patterning tool which defines a pattern for only a portion of a wafer. However, the terms “photomask” (or more generally “mask”) and “reticle” are frequently used interchangeably in modern parlance, so that either term can refer to a radiation patterning tool that encompasses either a portion or an entirety of a wafer. For purposes of interpreting this disclosure and the claims that follow, the term “reticle” is utilized generically to refer to any radiation patterning tool, inclusive of tools which define a pattern for only a portion of a wafer and tools which define a pattern for an entirety of a wafer.
Reticles contain light restrictive regions (for example, totally opaque or attenuated/half-toned regions) and light transmissive regions (for example, totally transparent regions) formed in a desired pattern. A grating pattern, for example, can be used to define parallel-spaced conductive lines on a semiconductor wafer. As discussed previously, the wafer is provided with a layer of radiation-sensitive material (such as, for example, photosensitive resist material, which is commonly referred to as photoresist). Radiation passes through the reticle onto the layer of photoresist and transfers a pattern defined by the radiation patterning tool onto the photoresist. The photoresist is then developed to remove either the exposed portions of photoresist for a positive photoresist or the unexposed portions of the photoresist for a negative photoresist. The remaining patterned photoresist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as, for example, ion implantation or etching relative to materials in the wafer proximate the photoresist.
Advances in semiconductor integrated circuit performance have typically been accompanied by a simultaneous decrease in integrated circuit device dimensions and a decrease in the dimensions of conductor elements which connect those integrated circuit devices. The demand for ever smaller integrated circuit devices brings with it demands for ever-decreasing dimensions of structural elements, and ever-increasing requirements for precision and accuracy in radiation patterning with reticles.
FIG. 1 shows a flow chart illustrating a typical process utilized for creating a pattern for a reticle. At an initial step 10, a preliminary design is created for the reticle and verified. The creation of the design begins with provision of a desired pattern which is ultimately to be formed in photoresist. Subsequently, the design is created for the reticle which will roughly produce the desired pattern on photoresist from radiation passed through the reticle. The design is rough in that it largely ignores effects of interference on radiation passing through the reticle.
After the design is believed to be complete, (i.e., once it is believed that all patterned features which are to be patterned in photoresist with the reticle are represented in the design) the design is submitted to a verification process to confirm that the design is complete.
After the design has been created and verified, it is subjected to optical proximity correction (shown as step 20 in FIG. 1). The optical proximity correction takes into account various interference factors that influence radiation passing through a reticle (i.e., constructive and destructive interference effects that result from passing radiation through patterns having dimensions on the same order as the wavelength of the radiation, or smaller). The optical proximity correction can be utilized to correct all parts of the design, or only some parts of the design. In other words, the optical proximity correction can be applied to only some portions of a design, while other portions are not optical proximity corrected. Typically there will be a verification step following the optical proximity correction.
The steps of generating a design from a desired pattern which is to be provided in photoresist, verification of the design, optical proximity correction, and verification of the correction, are typically accomplished primarily through the use of software. A suitable software package which can be utilized for one or more of the steps is HERCULES™/TAURUS OPC™, which is available from Avant! Corporation™.
The optical proximity correction creates a dataset which is subsequently translated into a pattern formed on a reticle. The process of translating the dataset into a pattern on the reticle is frequently referred to as taping the pattern onto the reticle. In such context, the terms “tape” and “tape out” refer to a process of transferring the dataset to appropriate hardware which writes a pattern represented by the dataset onto the reticle. The process of writing onto the reticle can be accomplished by, for example, laser writing and/or electron-beam writing methodologies. The step of taping the pattern onto the reticle is shown in FIG. 1 as step 30.
After the pattern has been formed on the reticle, the reticle can be utilized for patterning radiation in semiconductor fabrication processes. FIG. 2 illustrates an exemplary apparatus 40 in which a reticle is utilized for patterning radiation. Apparatus 40 comprises a lamp 42 which generates radiation 44. Apparatus 40 further comprises a reticle 46 through which radiation 44 is passed. A semiconductor substrate 48 having a radiation-sensitive material 50 thereover is illustrated associated with apparatus 40. The radiation passing through reticle 46 impacts radiation-sensitive material 50 to form a pattern within the radiation-sensitive material. The process of forming a pattern in a radiation-sensitive material with a reticle can be referred to as a printing operation. For purposes of interpreting this disclosure and the claims that follow, the term “taping” will refer to a process of forming a pattern on a reticle, and the term “printing” will refer to a process of forming a pattern on a radiation-sensitive material utilizing the reticle.
Reticle 46 typically comprises an opaque material (such as chrome) over a transparent material (such as a glass). Reticle 46 has a front side where the pattern is formed as features (or windows) extending through the opaque material, and has a back side in opposing relation to the front side. The shown reticle has two opposing sides 45 and 47, and in practice one of the two sides would be the front side (typically side 45) and the other would be the back side. In some applications features can be printed on both the front side and back side of the reticle.
As discussed above, reticle 46 will typically have a pattern with dimensions on the order of the wavelength of the radiation passing through the reticle, or smaller. Accordingly, various interference effects can occur as the radiation passes through the reticle so that the radiation exiting the reticle will transfer a pattern somewhat different than the pattern of the reticle. Such is illustrated diagrammatically in FIG. 3. Specifically, FIG. 3 illustrates an exemplary pattern 60, which can be desired to be formed in a radiation-sensitive material, and illustrates an approximation of a pattern 70 which would be formed in a reticle to generate the pattern 60. Pattern 70 is referred to as an approximation because the pattern is a qualitative representation of the type of pattern utilized in reticle for generating pattern 60, rather than a quantitative representation.
The FIG. 1 process can, for example, start with a pattern identical to pattern 60 being provided at the design step (10) of the reticle fabrication process, and such design would then be converted to the shape 70 during the optical proximity correction (20) step.
FIGS. 4 and 5 illustrate exemplary designs which can be desired to be formed in radiation-sensitive materials, and illustrate the reticle patterns commonly utilized to create such designs. Referring initially to FIG. 4, a radiation-sensitive material 80 is illustrated in top view, and a design is formed within the material comprising a plurality of features 82. The shown features 82 are circular in patterned dimension, and can be utilized, for example, in forming contact openings.
Features 82 can be desired to be identical in printed dimension relative to one another. The printed dimension of features 82 corresponds to the shape printed on a surface of radiation-sensitive material 80 during a photolithographic process (i.e., to a pattern of radiation formed on the surface of the radiation-sensitive material as the radiation passes through a reticle), as opposed to a depth of the features. The printed dimension is a circumference of the shown circular features.
FIG. 4 also illustrates a reticle substrate 84 comprising a pattern associated therewith which includes a plurality of identical elements 86. The elements 86 are in a one-to-one correspondence with the features 82 formed in the radiation-sensitive material. Further, each of elements 86 is approximately square in shape. In operation, radiation is passed through reticle 84 to form the pattern of printed images 82 on radiation-sensitive material 80. Regions 86 of the reticle can be either more transparent to radiation than surrounding regions of the reticle, or can be less transparent, depending on whether the radiation-sensitive material corresponds to a positive or negative material. If elements 86 are more transmissive to radiation than surrounding regions, the elements 86 can effectively be windows which allow radiation to pass through those specific regions of the reticle.
Referring to FIG. 5, a radiation-sensitive material 90 is illustrated in top view, together with a pattern comprising features 92 and 94 that is desired to be formed in the material. Feature 92 extends along a length 93, and feature 94 extends along a length 95. The lengths 93 and 95 are not parallel to one another in the shown embodiment, and in fact are substantially orthogonal to one another. Accordingly, features 92 and 94 can be considered to extend vertically and horizontally, respectively, relative to one another.
Features 92 and 94 can be desired to be identical in printed dimension relative to one another. The printed dimensions of features 92 and 94 correspond to the shapes printed on a surface of radiation-sensitive material 90 during a photolithographic process of forming features 92 and 94, as opposed to a depth of the features. The printed dimensions of the shown oblong features include length and width dimensions.
A reticle 96 is shown comprising a pair of elements 98 and 100. Reticle 96 can be utilized for generating the pattern associated with radiation-sensitive material 90. Specifically, the rectangular-shaped elements 98 and 100 can be utilized for forming the elongated shapes 92 and 94 as radiation is passed through reticle 96 and patterned with regions 98 and 100.
Various problems can occur in utilizing reticles to pattern radiation during a printing process. Such problems can include differing attributes of a radiation-sensitive material in different regions where features of a pattern are to be formed, and/or aberrations associated with radiation utilized during a printing process. An exemplary aberration associated with radiation can include astigmatism. In the exemplary processing described with reference to FIG. 5, it is desired that feature 92 and feature 94 be identical to one another (except that the features extend in different directions). However, if astigmatism occurs during the printing of features 92 and 94, such can cause different levels of exposure of the radiation-sensitive material associated with one feature relative to the exposure of the material associated with the other feature, which can cause the resulting features to be non-identical relative to one another.
FIG. 6 illustrates another aberration that can be associated with the radiation. Specifically, FIG. 6 illustrates a cross-sectional view of a fragment 105 of semiconductor construction during a printing process. The construction comprises a semiconductor substrate 110 having a layer 112 formed thereover.
In describing the various applications of the invention which follow, it is useful to utilize the term “substrate” to refer to various supporting structures, and combinations of supporting structures. Accordingly, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.
Layer 112 covers only a portion of the substrate. A radiation-sensitive material 114 extends over substrate 110 and layer 112. The radiation-sensitive material has a varying topography, and specifically has a lower surface over regions of substrate 110 that are not covered by layer 112, and a higher surface over the regions that are covered by layer 112.
Regions 116 and 118 are provided to diagrammatically illustrate areas where features are to be printed into material 114. The dashed lines of regions 116 and 118 are to be understood as locations relative to material 114, rather than as structures extending upwardly from material 114. The feature printed at location 116 will ideally be identical to the feature printed at location 118. However, such typically doesn't occur in practice.
Region 116 is associated with a lower portion of material 114 and region 118 is associated with a higher portion of the material. Radiation passing through the reticle should be focused at an upper surface of the material 114 that is to be patterned. However, since the upper surface of material 114 varies in height, the radiation cannot be simultaneously focused at both the location of feature 116 and the location of feature 118. Accordingly, the radiation is focused at an intermediate location, with a focal point of the radiation being illustrated diagrammatically by dashed line 120. Since the radiation is focused above the location of feature 116 and below the location of feature 118, there can be an image size difference at the location of feature 118 relative to the location of feature 116.
FIG. 7 illustrates various attributes that can be associated with a radiation-sensitive material. More specifically, FIG. 7 illustrates a fragment 125 comprising a semiconductor substrate 130, a layer 132 over a portion of the semiconductor substrate, and a radiation-sensitive material 134 formed over semiconductor substrate 130 and layer 132.
Radiation-sensitive material 134 comprises a thickness 135 which is substantially uniform over substrate 130 and layer 132, but which increases to a thickness 137 at a location where an elevational height of the material 134 changes due to layer 132. The change in thickness of material 134 can be considered a change in an attribute of the radiation-sensitive material, and such change can influence formation of a pattern within material 134.
A series of regions 136, 138, 140, 142, 144 and 146 are illustrated relative to material 134 where patterned features are to be formed within the material. It is noted that regions 136, 138, 140, 142, 144 and 146 are designated by dashed lines extending upwardly from region 134. The dashed lines are utilized to indicate where the regions will be formed relative to material 134 and not to indicate that any structures are extending above material 134. Each of regions 136, 138, 140, 142, 144 and 146 will ideally be identical to one another when the features are formed to extend through material 134. However, the varying topology of an upper surface of material 134 creates three regions (labeled as 129, 131 and 133) that ideally are separately analyzed relative to one another. Region 131 has thicker resist than the other regions; and regions 129 and 133 are optimized at different focal elevations relative to one another.
Since the resist is thicker under feature 140 than under the other features, the feature 140 will not be identical to the other features if subjected to identical processing as the other features. For instance, if feature 140 is printed to have an identical width as the other features, then the opening formed relative to feature 140 may actually be narrower at the base of the opening than are the openings formed relative to the other features.
Semiconductor wafers can have numerous regions where radiation-sensitive material has different attributes than other regions due to, for example, the topography of the material. Further, numerous aberrations can be present in light utilized for printing a pattern to photoresist. Presently, various defects introduced by aberrations in radiation and/or by differing attributes in radiation-sensitive material are frequently not addressed during a process of forming a reticle. However, as integrated circuit devices become smaller it becomes increasingly desirable to compensate for problems induced through aberrations in radiation or differing attributes of radiation-sensitive material. Accordingly, it would be desirable to develop methodologies for addressing such problems.